1. Field of the Invention
The present invention relates to steam turbine rotors, and particularly to a steam turbine rotor suitable for a steam turbine used in a large scale power generation plant or a gas turbine-combined power generation plant.
2. Description of Related Art
A turbine rotor of a common steam turbine is in a severe corrosive environment because normally, its low pressure stages (e.g., the low pressure final stage L-0 to the stage L-4 on a higher pressure side) are positioned under a wet steam condition or a dry and wet alternate condition, where dry steam and wet steam by turns exists.
In general, a low alloy steel such as a 3.5% Ni steel and a 1% CrMoV steel is adopted as a rotor material in low pressure stages of a steam turbine taking its mechanical strength, toughness, and large piece forgeability into account. Unfortunately, however, since its corrosion resistance is not necessarily high, long-time use of a low alloy steel in a plant can cause a corrosive medium to accumulate in the gaps between blades and blade implant parts of a rotor and result in stress corrosion cracking (hereinafter referred to as SCC).
Also, since blades with a long blade length (long blades) are employed in the low pressure final stage L-0, high centrifugal stress occurs at the blade implant parts. In the steam turbine of a combined power generation plant, in particular, variations in, and repeated application of, centrifugal stress accompanying starting/stopping operations can reduce low cycle fatigue life (hereinafter referred to as LCF life) of the steam turbine rotor in a corrosive environment.
Techniques to enhance the reliability of a turbine rotor used in the low pressure final stage L-0 of a steam turbine include those described in Patent Literatures 1 and 2, for example.
Patent Literature 1 (JP 2001-50002 A) discloses that a 12Cr steel with high corrosion resistance is employed as a rotor material used in the low pressure final stage L-0. Also, Patent Literature 2 (JP 2006-307840 A) discloses that susceptibility to SCC is reduced by reducing the yield strength of a rotor material in the low pressure final stage L-0 to the stage L-2 in such a way that the yield strength is lower toward the high pressure side.
As described above, major conventional problems related to low pressure turbine rotors of steam turbines are how to improve LCF life in the low pressure final stage L-0 in a corrosive environment and how to reduce SCC susceptibility in the low pressure final stage L-0 to the stage L-4.
Meanwhile, in recent years, blades in the low pressure final stage L-0 have been getting longer; blades with a length equal to or longer than 1250 mm can be employed at 3600 rpm, for example. Also, with this trend toward longer blades in the low pressure final stage L-0, blades in stages L-1 and L-2 which are closer to the high pressure side than the low pressure final stage L-0 are also becoming longer. This poses a requirement of improving LCF life in a corrosive environment even in such stages as L-1 and L-2, where it has not been much of a problem.
Moreover, in the stages L-1 and L-2, which are closer to the high pressure side than the low pressure final stage L-0, reducing SCC susceptibility is even more necessary. This is because the temperature and SCC susceptibility in the stages L-1 and L-2 are higher than those in the low pressure final stage L-0.
However, neither the JP 2001-50002 A nor JP 2006-307840 A mentioned above sufficiently discloses any materials or mechanical strength appropriate to reduce SCC susceptibility and, at the same time, to improve LCF life in stages L-1 and L-2, which are closer to the high pressure side than the low pressure final stage L-0.